There are many applications for active pixel image sensors, including scientific, as well as commercial and consumer applications. Active pixel sensors, for example, as described in U.S. Pat. No. 5,471,515, integrate the readout sensor as part of the pixel electronics. The special techniques of active pixel sensing allow using a semiconductor family formation process which is compatible with CMOS, e.g., NMOS. This technique enables the readout electronic to be integrated on the wafer using a similar process. The result is a high performance sensor with high quantum efficiency and low dark current.
One example of an application for active pixel sensors is in guidance systems in spacecraft. These guidance systems determine spacecraft attitude by matching an observed star-field to a star-catalog. These celestial star-tracker systems should be small in mass and power consumption, be radiation hard, have a high fill factor, and high sampling resolution. Since a star-tracker centroids an intentionally blurred star image, the effect of pixel geometry on the centroiding algorithm should be minimal. An active pixel sensor-based system can reduce mass and power consumption and radiation affects compared to a CCD-based system.
Spacecraft star-tracker systems may be required to image sections whose images vary by 10.sup.8 ratio relative to one another, e.g. the difference in brightness between the brightness of a nearby planet and the brightest stars. As result, the dynamic range of these devices becomes a crucial issue. Prior suggested solutions for widening the dynamic range of active pixel sensors fall into three basic categories: compressing the response curve, clipping the response, and control over integration time. The first two methods result in loss of some kind of detail in the image. This has led the inventors to consider the third--this can be done either externally or internally.
External control over integration time is generally preferred, and can be done either globally or locally. Global control over integration time has been achieved via electronic or mechanical shuttering, as well as by other means. The inventors found, however, that global control does not work well when viewing a scene that itself includes a wide dynamic range. This is because part of the sensor might be saturated or exposed below its minimum threshold, resulting respectively, in white or black patches in the picture. For example, when using a charge-coupled device (CCD) system, a single integration period for the entire sensor is normally required. This means that either a bright star is properly exposed, and a dim star is lost in the noise; or, the dim star is properly exposed, but the bright star is saturated and useless for centroiding.
Attempts have been made to implement local exposure control in active pixel sensors. However, these solutions have generally resulted in reduced fill-factor, which is not appropriate to many applications. Fill-factor is the ratio of the pixel area that is responsive to light, divided by the total area of the pixel. A low fill-factor will result in reduced resolution and/or excessively large arrays.
The present inventors have recognized that local exposure control would be desirable since it would permit a different integration period for different areas of the sensor. In star-tracker applications a different integration period for each star would make it possible to sense both bright and dim stars at the same time.
While the above has described use with stars, it should be understood that any variable image could similarly be imaged. According to one aspect of the invention, each individual pixel can be reset in an active pixel sensor (APS) pixel. This is accomplished using two reset transistors instead of a single reset transistor as in prior APS pixel circuitry. These two reset transistors are coupled to column reset and row reset lines. Both column and row reset lines must be on before the APS is reset. This enables individual control over the reset, and consequently the integration time, for each APS. Additional row and column controls are required to achieve individual pixel reset onto the image sensor. In addition to the conventional row-read control, a row-reset control is provided. Likewise, a column reset control unit is added to the conventional column-readout control. The column and row-reset controls in the invention are used to activate the column reset APS's individually.
The invention is a significant improvement over prior individual pixel reset APS designs. For example, some of these prior APSs used a second transistor in series with the row-reset transistor, activated by a vertical column-reset signal. This design may introduce reset anomalies when used in CMOS readout circuits for infrared focal-plane-arrays. It is believed this is due to charge pumping from the output node to the reset drain. An object of the present invention is to overcome these disadvantages and to provide an active pixel sensor with individual pixel reset which allows control over the integration time of each pixel individually.
The individual pixel reset (IPR) APS of the invention can be used to achieve a very wide dynamic range. Dynamic range can be modified by changing the integration time, which is the time that the pixel is exposed to incoming light. Integration time begins with a reset signal and ends with a sample signal. In prior APSs having global control, pixels are all reset and sampled at the same time. The present invention determines the reset time for each pixel individually. This allows each pixel to have a different integration. Reset is commenced by simultaneously activating both column and row reset control lines for a particular pixel at the same time. Sampling is accomplished at the same time for all the pixels. Thus, the time between reset and sample for each pixel will vary as desired simply by varying the reset time for each pixel.
The invention adds only a single additional transistor in each pixel as compared to previous (APS) designs. Because of this, the effect on fill-factor is minimal. This is in contrast to previously reported APS designs that require circuitry that results in large reduction in fill-factor to achieve local exposure control.
In accordance with the invention, different integration times can be set according to the intensity of the incident light. This allows the APS to have an extremely wide dynamic range. By sensing the light levels for each pixel in real-time and adjusting the integration times accordingly, the APS can accommodate light intensities which vary spatially, as well as temporally, over a wide dynamic range. Also, the minimum integration time can be varied according the illuminance level of different pixels. In contrast, the minimum integration time is constant in CCDs and in conventional APSs. This enables viewing details in high illumination areas, that would otherwise be lost using prior compression-like solutions for widening the dynamic range.